1. Field of the Invention
The present invention relates to an X-ray detector used to detect X-rays in an energy-dispersive X-ray spectrometer (EDS) that is mounted in a transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray fluorescent analyzer (XRF), or other similar instrument and, more particularly, to a technique for reducing the background of X-rays detected by a silicon drift X-ray detector (SDD).
2. Description of Related Art
A PIN detector consisting of silicon (Si) doped with lithium (Li) as shown in FIG. 9 has been available as a semiconductor X-ray detector used in EDS. The semiconductor device of FIG. 9 is a PIN diode to which a reverse bias is applied. The gate of a field-effect transistor (FET) is connected with one electrode of the PIN diode. When X-rays produced from a sample impinge on the PIN semiconductor, ion pairs whose number corresponds to the energy of X-ray quanta are created. The generated ion pairs are separated to thereby induce an electron avalanche by the reverse bias voltage applied to the semiconductor. The ion pairs are attracted toward the electrode. A current variation signal having an amplitude proportional to the energy of the X-ray quanta impinging on the PIN diode is extracted by varying the potential at the FET gate electrode. When X-rays are detected with a PIN detector, it is necessary to cool the PIN diode and FET with liquid nitrogen.
The SDD is a relatively recently developed X-ray detector permitting high count-rate measurements of X-rays in contrast with PIN detectors. The SDD is greatly different in structure from the PIN detector. The SDD is now briefly described by referring to FIG. 5, which schematically shows the structure of the X-ray detection device of the SDD. FIG. 5 is a partially cutaway view to facilitate understanding of the cross-sectional structure.
X-rays to be detected enter the X-ray detection device from the side of the cathode (C) (from the lower side of the drawing). The FET forming a part of the PIN detector is a separate device wire-connected. The FET of the SDD is fabricated on the rear surface of the X-ray detection device as shown in FIG. 5. That is, the FET has drain (D), gate (G), and source (S) as its electrodes. These gates are arranged in this order from the inner side on the X-ray detection device.
In order to enable high count-rate measurements of X-rays, it is necessary to minimize the time (time constant) during which ion pairs created by the X-rays incident on the X-ray detection device are electrically extracted. To reduce the time constant, the electrical parasitic capacitance of the X-ray detection device must be reduced. In the SDD, the parasitic capacitance is reduced by reducing the size of the anode (A) of the X-ray detection device, and by arranging the FET integrally with the X-ray detection device. However, electrons produced by X-ray impingement can be well guided to the anode only if the size of the anode is reduced. Therefore, as shown in FIG. 5, multi-stage annular rings (referred to as field strips) (more rings are formed in practical instrumentation) are mounted. A successively changing negative potential bias is applied to the rings from the innermost ring closest to the anode toward the outwardmost ring. Electrons are moved along stepwise electric fields applied to the field strips. Finally, the electrons are concentrated into the anode. The anode is connected with the gate electrode of the FET. A current variation signal having an amplitude proportional to the energy of the X-ray quanta impinging on the SDD is extracted by varying the potential at the gate electrode of the FET.
As the thickness of the X-ray detection device is increased, it is necessary to increase the reverse bias voltage applied to each field strip accordingly. However, if this voltage is increased excessively, it is necessary to satisfy stringent protective conditions (e.g., the inside of the container accommodating the X-ray detection device must be kept at a high vacuum). To strike a compromise between the protective conditions and the performance, the thickness of the X-ray detection device is set to about 0.5 mm, which is smaller than the thickness of the PIN detector.
The SDD normally provides electronic cooling using a Peltier device. One feature of the SDD is that energy resolution comparable to that achieved by a PIN X-ray detector is obtained by providing only cooling using a Peltier device without depending on liquid nitrogen. Another feature of the SDD is that the whole detector, including the cooling mechanism using a Peltier device, is miniaturized and made lighter. Consequently, it is easy to mount the SDD in an apparatus.
FIG. 3 is a fragmentary perspective view of an SDD in which a Peltier device is mounted. FIG. 4 is a cross-sectional view of the SDD shown in FIG. 3. The SDD has an X-ray detection device 1 having a rear surface to which an FET 1a is coupled. An electrode terminal subassembly 2 has electrical wiring for electrically connecting the X-ray detection device 1 and the FET 1a. The SDD further includes a Peltier device 3 and a rear thermal conductor 4. The electrode terminal subassembly 2 assumes a frame-like form and is provided with an opening 2a. The terminal subassembly 2 is in contact with the outer peripheral portions of the X-ray detection device 1 and holds it. Accordingly, a gap is left between the rear surface of the device 1 on which the FET is disposed and the Peltier device 3. The Peltier device 3 is mounted as close as possible to the X-ray detection device 1 via the electrode terminal subassembly 2 such that the detection device 1 can be cooled in a short time.
JP-A-2006-119141 discloses a technique for correcting variations in dependence of the charge-voltage conversion rate of SDD. JP-A-2005-308632 discloses a technique using a Peltier device for cooling of a PIN semiconductor device.
As described previously, the thickness of the X-ray detection device of the SDD is about 0.5 mm, which is considerably thinner than the prior art Si (Li) semiconductor device. Therefore, the ratio of the X-rays reaching the Peltier device mounted behind the X-ray detection device after passing through the X-ray detection device to the X-rays incident on the detector is high. Because the Peltier device is made of a material containing rich amounts of elements of high atomic numbers, secondary X-rays of relatively high energies are produced from the Peltier device. There is the problem that the background increases because the secondary X-rays enter the X-ray detection device from the rear surface and become detected.
Another problem arises from the fact that the Peltier device is mounted close to the X-ray detection device. A large direct electric current flows through the Peltier device. The current produces a magnetic field. If magnetic parts are present nearby, the magnetic field is complicated. If the direct current varies or noise is superimposed on it, the magnetic field is made more complex. An X-ray detector mounted in a TEM or SEM is disposed as close as possible to the investigated sample on which an electron beam impinges, in order to enhance the sensitivity at which X-rays to be observed are detected. However, as shown in FIGS. 3 and 4, the Peltier device for cooling the X-ray detection device is also brought close to the sample. Therefore, the electron beam impinging on the sample is affected by the magnetic field produced by the Peltier device, thus adversely affecting the performance of the electron microscope. This problem is not serious where the SDD is mounted in an XRF. However, this problem becomes serious when the SDD is mounted in an electron beam apparatus, such as a TEM or SEM, for analyzing a sample by irradiating the sample with an electron beam and spectrally detecting characteristic X-rays emanating from the sample. This problem becomes more conspicuous in a case where the SDD is mounted in a TEM as described below.
FIG. 6 is a schematic diagram illustrating the manner in which an analysis is made using a TEM in which an EDS is mounted. A very strong magnetic field is produced between the upper and lower polepieces of the objective lens to focus the electron beam. In order to detect a maximum portion of X-rays produced from the sample in response to electron beam irradiation, it is necessary to bring the EDS as close as possible to the sample. However, if the EDS is brought closer to the sample, the front end of the EDS comes closer to the magnetic field. If the EDS is a silicon drift X-ray detector (SDD), the magnetic field produced by the cooling Peltier device disturbs the magnetic field of the objective lens of the TEM, adversely affecting the electron beam.
On the other hand, FIGS. 7 and 8 are schematic diagrams illustrating the manner in which an analysis is performed using an SEM in which an EDS is mounted. FIG. 7 shows a case in which the SEM has an out-lens objective lens. FIG. 8 shows a case in which the SEM has a semi-in lens objective lens. In the case of an out-lens objective lens, the magnetic field of the objective lens does not leak close to the location where the EDS is located. The semi-in lens objective lens permits the magnetic field of the objective lens to leak to the sample surface, thus finely focusing the electron beam. Where the magnetic field is allowed to leak to the sample surface in use, however, the purpose is to obtain a high resolution. Under this condition, the distance between the bottom surface of the objective lens and the sample surface is quite small. Therefore, it is impossible to bring the EDS close to the sample to such an extent that the leaking magnetic field is affected. Consequently, if the EDS mounted in the SEM is an SDD, the effect of the magnetic field produced from the Peltier device is weaker compared with the case where the EDS is mounted in a TEM.
That is, the problem arising from thinning of the semiconductor device of the SDD is common to both cases in which the SDD is mounted in TEM and SEM, respectively. The problem with the magnetic field produced from the Peltier device occurs when the SDD is mounted in a TEM.